1. Field of the Invention
Present invention relates to an optical sensor array which converts an optical image to electrical information including electrical current or voltage or digital data and specifically relates to an optical sensor array with improved performance. This invention further relates to a driving or sensing method and circuit for it applicable to the optical sensor array of the present invention and further relates to an optical array device by the optical array and it's periphery circuits and to an optical imaging apparatus using the optical array device and an optical imaging means.
2. Description of the Related Art
In a conventional two dimensional array of photo-transistors as shown in FIG. 1 (FIG. 5 of reference 1), emitters of the photo-transistors are connected to a word line and collectors of the photo-transistors are connected to a column line, and a word line is driven to select photo-transistors and signal from the selected photo-transistor (a sensor element) is read as a voltage change across a load resistor connected to each column line. In this optical sensor array, signal delay due to collector time constants is remarkable. The collector time constants comprise product of the load resistance and collector-base capacitance with Miller effect and product of the load resistance and collector-substrate and collector-isolation region capacitance. The effect of the collector-base capacitance is multiplied by a voltage amplification factor of the photo-transistor and this phenomenon is well known as Miller effect. In addition, leakage current at collector-substrate and collector-isolation region junction is added to collector-base leakage current and this effectively increases dark current of the photo-transistor.
Furthermore, collectors of photo-transistors commonly connected to a column experience common out put voltage swings at the load resistor attached to the column. This intermixes a stored electrical datum in each photo-transistor and as a result, reduces dynamic range of the array or contrast of a read-out electrical image. Furthermore, a lower illuminance optical image results a smaller current output from the photo-transistor and a higher resistance value for the load resistor is required for a reasonable voltage output. Then, the time response of the output voltage becomes slower and switching noise for selecting each photo-transistor superposes on the out put signal.
Thus, lower the illuminance of the optical image becomes, more difficult the capture of the optical image within a reasonable time and with reasonable noise becomes.
Reference 1: J. S. Brugler, et al., “Integrated Electronics for a Reading Aid for the Blind”, IEEE Journal of Solid-State Circuits, Vol. SC-4, No. 6, p. 304-312, December, 1969.
On the other hand, a one dimensional photo-transistor array as shown in FIG. 2 (FIG. 5 of reference 2) is disclosed. Different from the above reference 1, a collector drive and emitter sense method is adopted. Thus the delay by the above stated collector capacitance was avoided but any selection method of two dimensional array for a higher response or a smaller switching noise is not taught. Furthermore, collectors are left as electrically floating when they are not selected. Even if a two dimensional array were laid out from plurality of the one dimensional photo-transistor arrays by connecting one of the collectors of neighboring one dimensional arrays of the reference 2 each other, the connected collectors are left as electrically floating when they are not selected and electrical charge stored in the collector-base junction which is a function of detected light strength will be intermixed through the connection among the collector-connected photo-transistors. Thus, a two dimensional optical image with a high dynamic range will not be obtained. In the reference 2, any sensing method is not disclosed for a two dimensional array and if a load resistor like the reference 1 is connected between a reference potential and a line with which the emitters are connected, reading a darker optical image with reasonable voltage output will result adoption of a higher resistance value and then a slower response and worse signal to noise ratio.
Reference 2: Japanese Patent Disclosure Shou 60-198959, “Image Sensor”